1. Technical Field
The present invention relates generally to an improvement on an extrusion die for molding a honeycomb structure such as a catalyst carrier of a catalytic converter for internal combustion engines and a method of manufacturing the same.
2. Background of Related Art
FIG. 4 shows a catalyst carrier of a conventional catalytic converter used in an internal combustion engine exhaust system.
The catalyst carrier is formed with a honeycomb structure 8 consisting of latticed cell walls 81 defining cells 82. The honeycomb structure 8 is usually formed by extruding a ceramic material into a honeycomb shape, cutting it to a desired length, and drying it.
FIG. 5 shows a conventional extrusion die for forming the honeycomb structure 8 which includes material inlet ports 4 and latticed slits 92 communicating with the material inlet ports 4. The ceramic material is forced into the latticed slits 92 through the material inlet ports 4 to form the honeycomb structure 8.
In recent years, there is an increasing need for the honeycomb structure 8 to be decreased in thickness of the cell walls 81 for lightening and improvement of performance thereof. The thinning of the cell walls 81 of the honeycomb structure 8 requires reduction in width of the silts 92 of the extrusion die. The reduction in width of the slits 92 will, however, cause the resistance to extrusion of the material through the slits 92 to be increased, necessitating the use of a large extruder.
In order to avoid the above problem, it is effective to shorten the length of the slits 92 in the direction of extrusion. This, however, encounters a drawback in that the material injected from the material inlet ports 4 is extruded out of the die without spreading all over the latticed slits 92, thereby leading to the formation of defects in the honeycomb structure 8. Specifically, the formation of the honeycomb structure 8 without any extrusion defects requires a sufficient length of the slits 92 in the direction of extrusion.
Japanese Patent Second Publication No. 61-164 discloses an extrusion die, as shown in FIG. 6, for a honeycomb structure designed to alleviate the above problem. The extrusion die 902 has, as clearly shown in the drawing, latticed slits 92 of a stepped structure. Specifically, each of the slits 92 has a wide portion 93 leading to one of the material inlet ports 4.
This structure eliminates the lack of spread of the material in the slits 92, yet it allows the length of the slits 92 in the direction of extrusion to be decreased. This permits the width of the slits 92 to be decreased without increasing the extrusion resistance, resulting in a decrease in thickness of the cell wall.
The above extrusion die, however, has the following drawback.
The formation of the wide portions 93 of the slits 92 requires a complex manufacturing process, resulting in a great increase in number of machining processes.
The formation of the wide portions 93 wider than the slits 92 makes it difficult to machine the material inlet ports 4 and the slits 92 in a single steel plate. It is, thus, necessary to machine the material inlet ports 4 and the slits 92 in different steel plates and bond them into a single piece. This results in a rise in manufacturing cost of the honeycomb structure.